CN107635424B

CN107635424B - Shock-absorbing structure and helmet with same

Info

Publication number: CN107635424B
Application number: CN201680017968.2A
Authority: CN
Inventors: 詹姆斯·库克
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2015-02-04
Filing date: 2016-02-04
Publication date: 2020-12-18
Anticipated expiration: 2036-02-04
Also published as: US20180027914A1; EP3253243A1; GB201501834D0; EP3253243B1; CN107635424A; WO2016125105A1

Abstract

A shock absorbing structure comprising a unitary material formed as a stretch dominated hollow cell structure, and a helmet comprising such a structure as an internal impact resistant liner.

Description

Shock-absorbing structure and helmet with same

Technical Field

The present invention relates to a shock absorbing structure. In particular, the present invention relates to a hollow cell shock absorbing structure. More particularly, the present invention relates to a shock absorbing structure formed as a stretch dominated hollow cell structure. The invention also relates to a shock absorbing structure with a curved impact surface, such as a sports helmet or an aviation nose bumper, at least part of which is formed by a hollow cell shock absorbing structure, even more particularly a tension dominated hollow cell shock absorbing structure.

Background

Personal injury or object damage may occur when a person or object is impacted with sufficient force. Considerable effort has been expended in order to produce materials and structures that provide protection from potentially damaging and deleterious effects.

Impact protection is particularly important to prevent head injuries. The blow to the head can cause severe Traumatic Brain Injury (TBI). Brain trauma is a frequent result of a focused impact on the head, or a sudden acceleration/deceleration of the brain, or a combination of impact and motion. Traumatic brain injury can cause long-term problems and treatment regimens are limited.

One of the most common causes of head injury is in the participation of athletic activities. For example, a fall while riding a bicycle may cause the head to strike a firm, hard object or surface, such as a road surface or the like. To prevent injuries, helmet use in the morning is habitual or mandatory in many sports such as bicycles, motorcycles and horse riding, rock climbing, american football and winter or ice sports such as skating, hockey and skiing. Another common cause of head injury is the fall of an object on a building or construction site.

Sports helmets and hard hats are designed independently so as to be particularly suitable for their particular use. However, most or all helmets have common design elements, such as a hard outer shell (formed from a rigid thermoplastic or composite material) and a liner/liner that is softer than the outer shell but still strong enough to retain its shape when unsupported. In combination, the shell and liner act to absorb impact forces and help prevent such forces from being transmitted to the head and brain. Almost all helmets use expanded polystyrene as the energy absorbing liner. The expanded polystyrene is formed into a unitary structure of a desired shape (i.e., without gaps).

US patent US 3447163 describes and illustrates a safety or crash helmet for use by motorcycle riders and/or racing drivers. The helmet has an outer shell formed as a double-layered member, the double-layered members of the outer shell being connected to each other around the periphery of the outer shell by gently curved peripheral portions without sharp edges, and the spaces between the layers containing layers of honeycomb material, the cells of the honeycomb layer being filled with an energy-absorbing foam material.

US patent US 7089602 describes and illustrates a shock absorbing modular helmet having an impact time increasing layer on the outside of the hard shell to reduce the impact strength. These layers are made of a uniform and consistent shock absorbing polymer material, which is filled with air or a polymer structure. These shock absorbing layers can also be made and used as a separate, removable outer protective shield that can be mounted on a hard helmet.

The aesthetic patent US 6247186 describes and shows a helmet having a shell, an inner impact resistant layer shaped to match the rider's head, a protective shield over and integral with the shell, and a chamber surrounded by the shell and the protective shield that is placed in front for ventilation. The chamber has a webbing on its front side for preventing entry of foreign matter and has one or more internal channels communicating with the helmet interior space through channels. In use, fresh air flows through the channels and into the impact resistant layer.

Sports helmets and hard hats must typically be worn for extended periods of time, and the weight of the helmet is an important design consideration. In designing a helmet, there is typically a tradeoff between the overall weight (and shape and size) and the shock absorbing performance of the helmet. Increasing the amount of shock absorbing material will increase the overall weight of the helmet and may also result in an increase in the outer dimensions, which in turn can make the helmet relatively more cumbersome and uncomfortable to wear, especially if aerodynamic considerations are also important. Conversely, if the helmet has too little shock absorbing material, impact protection may be compromised.

Foams such as those used in helmets are often excellent energy absorbers because they have a long plateau stress and in most impacts this area is constant, so the stress can be directly converted to a force for providing a long plateau force. This means that all energy can be absorbed while keeping peak forces and accelerations low, minimizing brain damage. However, in an elliptical helmet, the area at impact is not constant.

This is generally done to provide a background for discussing the features of the invention in this specification where reference is made to patent specifications, other external documents, or other sources of information. Unless otherwise expressly stated, reference to such external documents is not to be construed as an admission that such documents, or such sources of information in any jurisdiction, are prior art, or form part of the common general knowledge in the art.

Disclosure of Invention

It is an object of the present invention to provide a range of shock absorbing structures optimised for improved shock absorption, or at least to provide the public or enterprise with a useful choice. It is a further object of the present invention to provide a series of optimised shock absorbing structures which can be used to reduce traumatic brain injury by reducing the effects of peak acceleration and forces on the brain and directing energy away from delicate areas of the brain, or at least to provide the public or enterprise with a useful choice. It is a further object of the present invention to provide a helmet formed at least in part from an optimised shock absorbing structure which helps to reduce traumatic brain injury by reducing the effects of peak acceleration and forces on the brain and directing energy away from vulnerable areas of the brain, or at least provides the public or enterprise with a useful choice. It is another object of the present invention to provide a method of optimizing a shock absorbing structure for improved shock absorption.

The term "comprising" as used in this specification and the indicative independent claims means "consisting at least in part of" and is intended as an inclusive rather than exclusive term. When interpreting each statement in this specification and the indicative independent claims that include the term "comprising," features other than that term or terms preceding that term can also be present. Related terms such as "include" and "comprise" are to be interpreted in the same way.

As used herein, the term "and/or" refers to "and" or ", or both.

As used herein, a noun followed by "(s)" refers to a plural and/or singular form of the noun.

Accordingly, in a first aspect, the present invention may broadly be said to consist of a shock absorbing structure comprising a unitary material formed as a stretch dominated hollow cell structure.

In one embodiment, substantially all of the cells of the hollow cell structure are 2D hollow cells.

In one embodiment, substantially all of the cells are aligned substantially out-of-plane.

In one embodiment, the cells are formed as micro-truss lattices.

In one embodiment, the cells are formed as a lattice structure.

In one embodiment, at least a plurality of the cells are configured as a mosaic.

In one embodiment, at least a plurality of the cells are configured to tessellate with a cell axis perpendicular to the surface or out-of-plane orthogonality.

In one embodiment, at least a plurality of the cells are hexagons.

In one embodiment, at least a plurality of the cells are triangles.

In one embodiment, at least a plurality of the cells are square.

In one embodiment, at least a plurality of the cells are a combination of octagons and squares co-located on a damascene pattern.

In one embodiment, a monolithic material having a relative density of substantially between 0.05 and 0.15 is formed.

In one embodiment, the cell shape, size, cell wall thickness, cell width and cell length may be freely varied with respect to each other.

In one embodiment, the ratio of cell wall thickness to cell length is very small.

In one embodiment, the cell walls have a maximum thickness of about 1 mm.

In one embodiment, the monolithic material is a polymeric material.

In one embodiment, the monolithic material is an elastomer.

In one embodiment, the monolithic material is elastoplastic and elasto-brittle.

In one embodiment, the monolithic material is nylon 11.

In one embodiment, the monolithic material is an ST elastomer.

In one embodiment, the hollow cell structure is manufactured by laser sintering.

In a second aspect, the invention may broadly be said to consist in a helmet comprising an internal impact resistant liner formed at least in part from a shock absorbing structure as claimed in any one of the preceding claims.

In one embodiment, the helmet further comprises an outer shell for substantially covering the inner impact resistant liner.

In one embodiment, the housing is formed at least in part from a composite material.

In one embodiment, the housing is at least partially formed of a thermoplastic material.

In one embodiment, at least one vent slot is formed in the housing.

In a third aspect, the invention may broadly be said to consist in a method of optimising a shock absorbing structure to improve shock absorption, comprising the steps of:

(i) selecting a material;

(ii) the material is formed into a predominantly stretched hollow cell structure.

In one embodiment of the method, substantially all cells of the hollow cell structure are 2D hollow cells.

In one embodiment of the method, substantially all of the cells are formed so as to be substantially aligned out-of-plane.

In one embodiment of the method, the cells are formed as micro-truss lattices.

In one embodiment of the method, the cells are formed as a lattice structure.

In one embodiment of the method, at least a plurality of the cells are formed for damascene.

In one embodiment of the method, at least a plurality of the cells are formed so as to be tessellated with a cell axis that is orthogonal to the surface or out-of-plane.

In one embodiment of the method, at least a plurality of the cells are formed to have a topology that propagates radially to the curved surface.

In one embodiment of the method, at least a plurality of the cells are formed as hexagons.

In one embodiment of the method, at least a plurality of the cells are formed in a triangle.

In one embodiment of the method, at least a plurality of the cells are formed as squares.

In one embodiment of the method, at least a plurality of the cells are formed as a combination of octagons and squares co-located on the damascene pattern.

In one embodiment of the method, the material is formed in such a way that the formed material has a relative density substantially between 0.05 and 0.15.

In one embodiment of the method, the cells are formed such that the cell shape, size, cell wall thickness, cell width and cell length can be varied freely with respect to each other.

In one embodiment of the method, the cells are formed such that the ratio of cell wall thickness to cell length is very small.

In one embodiment of the method, the cells are formed such that the cell walls have a maximum thickness of substantially 1 mm.

In one embodiment of the method, the bulk material is a polymeric material.

In one embodiment of the method, the monolithic material is an elastomer.

In one embodiment of the method, the bulk material is elastoplastic and elasto-brittle.

In one embodiment of the method, the monolith is nylon 11.

In one embodiment of the method, the bulk material is an ST elastomer.

In one embodiment of the method, the hollow cell structure is manufactured by laser sintering.

With respect to the above description it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention.

The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, where specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Accordingly, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Drawings

Other aspects of the invention will become apparent from the description, given by way of example only, and made with reference to the accompanying drawings, which show, by way of illustration, embodiments of the apparatus, and in which:

FIGS. 1A-C show a schematic of a single cell forming part of a porous solid, showing the junction j that the cell shares with adjacent cells and the struts s, the struts s forming the perimeter faces surrounding the cell; fig. 1A shows a bend dominated structure where the joints are locked and the frame bends when the structure is loaded, such as the tension dominated structure shown in fig. 1B and 1C, where the members carry tension or compression when loaded, providing higher modulus and strength.

FIGS. 2A and 2B show the relative modulus E/Es and the relative strength σ/σ in terms of the relative density ρ/ρ s₃A graph summarizing the difference between tensile and bending dominant structures.

Fig. 3 shows a top-down and side-up perspective view of a honeycomb-shaped hollow cell structure according to an embodiment of the present invention.

Fig. 4 shows a top view from directly above of the hollow cell structure of fig. 3.

Fig. 5 shows a portion of the periodic lattice of hexagonal cells, illustrating the locations of the joints/and struts s of the tension dominated structure.

Figure 6 shows a perspective view of one side of an internal impact resistant liner of a bicycle helmet, the internal impact resistant liner being formed of a hollow cell structure similar to that shown in figures 3, 4 and 5, the liner being shaped to follow and substantially conform to the top of a user's head.

FIG. 7 shows a perspective view from the front and back of the internal impact pad of FIG. 3 with an outer shell covering the internal impact pad and vent slots formed in the outer shell to allow air to circulate within the internal impact pad.

Figure 8 shows a schematic perspective view of the front and one side of a test station for testing hollow cell structure samples.

Fig. 9 is a graph showing Head Injury Criteria (HIC) and peak acceleration for a series of test samples.

Fig. 10 to 12 show test samples of honeycomb-shaped hollow cell structures of subsequent tests according to an embodiment of the present invention, each sample having a different relative density, fig. 10 showing brittle fracture at a relative density of 0.111, fig. 11 showing plastic work at a relative density of 0.143, and fig. 12 showing linear elastic deformation at a relative density of 0.25.

FIG. 13 shows a plot of energy per volume versus peak acceleration for a series of test materials and conditions.

FIG. 14 shows acceleration vs. time and force vs. displacement for test pieces formed of nylon 11, elastomer, and expandable polystyrene.

Detailed Description

Embodiments of the present invention will now be described with reference to the accompanying drawings. As outlined in the background section above, certain structures are known to be suitable for impact absorption. However, it is not fully understood how to structure it to optimize impact absorption properties. Examples of optimized structures to improve impact absorption are listed below. These can be used to form articles intended to reduce traumatic brain injury, such as bicycle helmets. Methods of optimizing structures to improve impact absorption are also described.

Previously, in evaluating energy dissipation in helmets or similar shock absorbing structures, it was assumed that cushioning foam was responsible for dissipating impact energy. The reaction force is determined by the compressive strength of the foam. The foam lattice is assumed to have a flat plateau compressive strength at its densification strain. However, if the compressed area is uniform over an area, the foam only provides the desired force-displacement curve. In curved structures such as helmets having a generally oval shape, the impact or crush zone is not constant, or planar: the contact area increases with displacement. This results in an increase in the reaction force. Furthermore, if the curved helmet surface impacts another curved surface, the force-displacement gradient will be further reduced. As a result, the foam cushion needs to be thicker to provide adequate energy absorption by keeping peak accelerations below safety legislation. The consistent plateau stress of the foam limits effectiveness to energy absorbing structures when used as curved structures (e.g., in helmets) due to the inherently curved contact surfaces. Another assumption is that the liner is formed as a unitary structure (i.e., no gaps).

As described below, a structure can be created that is relatively rigid and strong, otherwise the structure will be created as a unitary structure formed from, for example, foam, for a given relative density ρ/ρ s (where ρ is the density of the foam and ρ s is the density of the matrix material), and this allows more energy to be consumed per volume. Structures that provide initial high strength can also be created when the contact area is very small and has a gradual post-yield softening proportional to the increase in contact area.

This is achieved by forming a shock absorbing structure such as a tension dominated hollow cell structure, for example a micro-truss lattice or an out-of-plane honeycomb.

In this type of structure, the deformation mechanism involves a "hard" mode, such as compression and tension, rather than bending. For the same relative density as the foam, the tensile dominated structure has a relatively high modulus and yield stress. As will be discussed below.

In the stretch dominated hollow cell structure, yield stress occurs due to local plastic buckling and brittle failure of the struts. This is also called the bifurcation point, as the structure becomes unstable and then a post-flexor softening scheme is created.

At strain of densification: (_d) The stress rises sharply, which can be calculated from the following equation:

where ρ is the density of the structure and ρ s the density of the matrix material, where s is the relative density (or solid volume fraction) of the structural locking.

In addition to light weight and ventilation, the use of a hollow cell tension dominated structure as a shock absorbing structure has two potential major advantages. First, the dorsiflexion softening offsets the increase in area over which the elliptical helmet dissipates energy under more uniform plateau forces. Second, for a given yield stress, the relative density of the tensile dominated structure can be lower, providing greater densification strain and thus increasing the potential energy dissipated at the same displacement.

One particular form of stretch dominated structure is a porous solid. The porous solid is comprised of an interconnected network of solid struts or plates that form the cell edges and faces. In general, the mechanical properties of porous solids can be distinguished by bending (foam) and stretching (lattice) dominant mechanisms. Maxwell stability criteria are used to distinguish between bend and stretch dominant structures. The porous solid may be considered as a joint connected by struts that surround the face of the surrounding cell, as shown in fig. 1.

The material effect on the face stiffens the structure by a constant amount. In fig. 1A, when the frame is compressed, it has no stiffness or strength in the direction of the load. If the joint is frozen (locked), the frame in FIG. 1A will bend when the structure is loaded, and may be referred to as a bend dominated structure. In the tensile dominated structure in fig. 1B and 1C, the members are under tension or compression when loaded, providing higher modulus and strength. This is shown in fig. 2A and 2B, which are based on the relative modulus E/Es and the relative intensity σ/σ, versus the relative density ρ/ρ s₃The differences between the tensile and bending dominant structures are summarized. In the structure of fig. 1A and 1B, the structure is subject to self-stress, meaning that the struts are subject to stress even if the structure is not subject to external forces (as is common in fig. 1C). For example, if a vertical strut shortens, the other struts are pulled into compression.

Two major advantages of using a tension dominated structure as a shock absorbing structure are as follows: first, the retroflexion softening offsets the increase in the area of the elliptical helmet that dissipates energy under more uniform platform forces; second, for a given yield stress, the relative density of the tensile dominated structure can be lower, providing greater densification strain and thus increasing the potential energy dissipated at the same displacement. This is discussed in detail in appendix E.

In the embodiments described below, the shock absorbing structure is formed as a lattice-i.e. interconnected hollow cells. Furthermore, to simplify processing, a periodic lattice (i.e., regular shape and size of the cells) is described. Hexagonal cells are used because this shape has the largest number of sides and it will still tessellate-i.e. no second shape is required to fill the gap (e.g. a regular square will be inherent if a regular octagonal lattice is chosen). Hexagonal honeycomb cells, which have the highest number of cell walls for each pair and thus the lowest connectivity, have proven to be effective at high specific strengths.

Other shapes (e.g., triangles and squares) will tessellate, but have fewer sides. However, the number of sides has been shown to be positively correlated with the dissipated energy per unit mass (SAE) of the structure.

The types of lattice structures described above can be generally described as 2D hollow cell structures. In those mentioned in the present specification, this means a three-dimensional structure having structural units formed in a manner to have a depth, but as a result, the units will have a uniform or identical cross section at any position perpendicular to the viewing angle when viewed at an angle. That is, a cross-section taken at any location is the same as a cross-section taken at any other location. For example, a honeycomb cell structure viewed in plan or from above would provide a uniform cross-section of any depth of the cells. This can be converted into a curved shape, for example an oval shape as required to form a helmet. When viewed from any particular point inward toward the interior center, the cell will appear the same as if viewed from another point inward toward the interior center.

It should be noted that other types of structures formed as stretch dominated structures will provide the same advantages. For example, 3D stretch dominated structures such as truss structures or lattice-like structures may also be formed, which would provide the same impact absorption benefits.

As shown in fig. 3 and 4, the hollow cell stretch leader 1 used in the first embodiment of the present invention is a monolithic material forming a honeycomb structure. Preferably, the cells are hexagonal, such as hexagonal cells 2, for example those used in the hollow cell structure 1, thus forming a structure in which each cell wall is common to adjacent cells.

The grid formed by the hexagonal cells also provides a balance between the overall grid density (total amount of material), the placement/location of the cell wall material, and the empty space enclosed by the cell walls. That is, tessellation is achieved by distributing cell walls over a given planar or curved surface, forming subdivisions as uniformly as possible, without excessive focal area or excessively large uncovered area.

Hexagonal honeycombs can be regarded as tensile dominated structures by applying maxwell standards:

FIG. 4 shows a portion of a periodic lattice of hexagonal cells, demonstrating the joint of such a stretch dominated structurejAnd a supportsThe position of (a).

In practice, the honeycomb structure will be exposed to in-plane and out-of-plane loading when subjected to an impact. Stretch dominated structures such as hexagonal hollow cell structures 1 are typically used in planar or sheet form, planar or curved, and the impacts received by the hollow cell structures have a major force component that is directly inset into a plane perpendicular to the point of impact. That is, in the direction opposite to out-of-plane arrow 3 in fig. 1. However, as noted, there is often a force component at an angle to it, and the theory behind this is discussed in detail in appendix C.

The impact absorption properties of stretch dominated structures such as the hollow cell structure 1 are determined by the materials used to form the structure and the specific geometry of the structure: i.e., cell size, cell wall thickness, cell width and cell length as shown in fig. 4.

If used in a shock absorbing structure such as a helmet, the lattice is designed so that the axial portions of the cells are always perpendicular to the surface of the head. This is important because the compressive strength of the honeycomb body weakens as the angle of impact increases away from the perpendicular to the axial portion of the cell.

If the cell size is known, the value of the relative density (or solid volume fraction) of the hollow cells can be calculated using the equation shown below:

for the particular embodiment of the honeycomb structure 1 according to the invention, h is equal to 1, θ is 30 °, and the ratio of the cell wall thickness (t) to the cell length (l) is very small.

The hollow cell structure 1 is suggested for use in a bicycle helmet. The materials used to make the hollow cell structure 1 in this embodiment are nylon 11 and ST elastomer. This is a readily available material which is light, easily formed and extensible and therefore suitable for or at least similar to the type of material used for the mass production of helmets.

The hollow cell structure 1 is manufactured by an incremental manufacturing technique. This process is briefly described in appendix B.

The tests were carried out in accordance with the detailed description of appendix A and appendix D in order to determine how the relative density of the honeycomb hollow cell structure 1 (this type of structure is also referred to as "out-of-plane honeycomb") which changes when subjected to an impact test affects the hollow cell structure 1. The relative density was varied between 0.1 and 0.33 by varying the cell size from a minimum of 6mm and a maximum of 20m, while maintaining the wall thickness at a constant value of 1mm, as shown in appendix A.

The results show that for the reasons outlined in the "impact test results" section of appendix a and appendix D, the acceptable range of optimum relative densities is between 0.125 and 0.175 for this material and for the particular cell/lattice size and shape used during testing. The results show that the cell size, cell wall thickness, cell width and cell length can be varied relatively freely, and that the structure will provide optimized impact absorption performance as long as the relative density is between 0.03 and 0.17.

As discussed in the background section, helmet design is typically a tradeoff between the overall weight and shock absorption performance of the helmet. Based on the results of the detailed tests in appendix a and appendix D, a helmet using a helmet 5 as shown in figures 3 and 4 constructed in a structure identical or similar to an internal impact pad 7 (formed as a hexagonal hollow cell tensile dominant structure) made of nylon 12 or similar material covered by an outer shell 6 will provide a lightweight structure, particularly BS EN 1078, capable of meeting and exceeding the relevant standards for impact absorption. The results of the tests show that the EPV of the elastoplastic honeycomb is 3 times higher than that of a typical expanded polystyrene helmet. This is clearly illustrated by the curves of the experimental results shown in figures 13 and 14. The reasons can be summarized as follows:

the stretch dominated structure relies on a "hard" mode of deformation through compression and tension. As the stress response softens with increasing area, a long plateau force is achieved.

The tensile dominated structure has a higher specific strength for the same relative density, so the densification strain may be increased as the relative density of the tensile dominated structure is lower.

All tensile dominated structures have similar mechanical response. Thus, the shock absorbing structure may be formed of any suitable material and in any shape and size (e.g., all honeycomb topologies and materials) and still provide the advantages described above.

As mentioned above, the 2D hollow cells referred to in this specification, which means a three-dimensional structure having structural cells formed in a manner to have a depth, but as a result, the cells will have a uniform or identical cross-section at any position perpendicular to the viewing angle when viewed at an angle. That is, a cross-section taken at any location is the same as a cross-section taken at any other location. For example, a honeycomb cell structure viewed from the plane or from above would provide a uniform cross-section of any depth of the cells. It should also be noted that when a structure is referred to as "stretch dominated," this is in accordance with maxwell standards as outlined herein. It should also be noted that the phrases "relative density" and "solid volume fraction" have essentially the same meaning and are used interchangeably in this specification.

Appendix A-test methods and results

A series of hollow cell structures were fabricated from nylon 12 by selective laser sintering. The area of the cross section of each sample was 100cm²And the depth is 10 cm.

The drop test was used for testing. An aluminum alloy head mold weighing 7.21kg was dropped vertically onto the sample over a distance of 1.48 m. The sample was released using the handle and oriented using the guide plate on the test stand shown in fig. 8.

And a single-axis accelerometer is placed at the position of the mass center of the head model. The sampling rate in LabView was set to 1000 Hz.

It was found that relative densities between 0.125 and 0.23 would pass the relevant british standard in this test. At a relative density of around 0.15, the peak acceleration drops to 53% of the maximum threshold. As the relative density increases, the consistency of the results increases. The head injury criterion HIC was calculated by post-processing software Diadem, the head performance index value having a trend line similar to the peak acceleration.

According to the results (see table below, and fig. 9), AIS grade 3 (moderate) damage will occur for honeycombs with a relative density of 0.15, while the risk of severe head damage will be minimized to 5%. At british standard thresholds, AIS grade 4 (severe) injuries can occur, while the risk of severe injuries will increase to 24%.

The results show that the optimum relative density is between 0.125 and 0.175. Fig. 10, 11 and 12 show deformation modes with relative densities of 0.111, 0.143 and 0.25, respectively. Sample 8 showed a fracture pair with plastic deformation, which seems to be the most efficient form of deformation. For a relative density of 0.33, no permanent deformation mode was observed, indicating that only linear behavior occurred.

The Head Injury Criteria (HIC) are used to measure the likelihood of head injury from impact measurements. HIC is the most widely used predictor for brain injury calculation at present. HIC plots translational acceleration of the head against duration at this acceleration.

Hexagonal cell structure optimization

The impact properties of the elastomer and elastoplastic hexagonal honeycomb structures out of plane were studied by varying the relative density and cell height. These results were compared to foam sections cut from conventional bicycle helmet liners.

In a drop hammer system, a rimstone shape is used as the impact projection. Bee productThe geometric parameters of the socket vary within each impact range: cell width, cell wall thickness, cell height, and cell pad. Each honeycomb sample had a constant cross-sectional area of 100mm x 100mm and was positioned so that the cell walls were always axial along the z-direction as shown in the bench diagram. Polycarbonate sheets of 0.375mm, 0.5mm, 1mm and 2mm thickness were placed on top of the sample to represent the shell. To test the expandable polystyrene material, the helmet was divided into nine sections, each having approximately the same surface area as the honeycomb structure. Since the expandable polystyrene portion is not flat, the rigid polyethylene filler is molded to provide a curved support. The impact speed of the rimstone anvil with the mass of 5 kilograms is 4.57 +/-0.1 ms^-1。

The energy absorption test of the 1078 standard was repeated using a drip tower. A high speed photographic technique of 2000 frames/second was used to track the impact force of the anvil and record the response of the honeycomb structure. A 15 mm lightweight trigger was used to trigger the high speed camera before impact. The impact anvil is connected to a rod suspended in a rigid cage, ensuring that it can only travel in the z-axis. When the anvil and the rod impact, the rigid cage will continue to move freely until it comes into contact with the damper.

Since the foam theoretically has a constant plateau stress, the force increases in proportion to the displacement.

Commonly used head injury criteria are used to analyze the likelihood of potential brain damage. Head Injury Criteria (HIC) is a measure of the magnitude and duration of deceleration, above 750-1000s-g²⁵Representing 16% risk of serious injury. The following table shows the "best" structures s found by testing different material samples.

The foregoing table lists HIC values for 2, 5 and 10 ms EPS foams, elastomeric honeycombs and elastoplastic (PA 11) honeycombs. These three changes show an exceptionally low HIC value for PA11, with a minimum HIC value of 44. Higher HIC values are expected when the helmet is adjusted to +50 ℃ and-20 ℃ as given by safety legislation. The relationship between the magnitude and duration of acceleration has proven to be important in causing brain damage. The Wien State Tolerance Curve (WSTC) is used to plot the magnitude of impact duration, and the red threshold curve describes the lethal tolerance limit of the brain. Standard impact curves for elastomers, PA11 honeycomb and EPS foam are plotted on WSTC. All curves are below the lethal threshold. Interestingly, EPS is always furthest from the extremes, indicating that a slow gradual force-displacement curve may be more effective in preventing brain damage. However, the acceleration duration is almost twice as long as that of PA 11.

Energy absorption per volume (EPV) is the amount of kinetic energy lost from projection on the maximum displaced volume of a structure, measured by using digital image correlation. At higher EPVs, the structure consumes or stores more kinetic energy over the same volume. This is also equivalent to the integral of the stress-strain curves used by Gibson and Ashby to create the continuous energy absorption diagram.

For EPVs equivalent to EPS, the optimum peak acceleration is reduced by more than 60%, highlighting the applicability of this type of structure and material for helmets. It is clear that elastoplastic (nylon 11) honeycomb structures with relative densities above 0.15 are too stiff and respond with extremely high peak accelerations, for example 650g peak acceleration is obtained at 0.33 density. However, at about 0.1 density (blue), the peak acceleration is similar to EPS, but with three times greater EPV. The response of the nylon 11 honeycomb was plastic bending and cell wall rupture.

Laser sintered PA 12 showed strain rate and temperature dependence confirming that the polymer was amorphous. The energy density is higher than 0.37J/mm²The mechanical properties deteriorate at low, medium, and high strain rates. The/3 transition may be in about 1000s^-1And found at-50 ℃ at TgAnd/3 there is a natural temperature dependence between.

The elastomer and elastoplastic materials are formed into a honeycomb structure by additive manufacturing techniques as described in appendix B. Under safety legislative impact conditions and the structure was impacted out of plane and compared to the expanded polystyrene section cut out in the bicycle helmet. The elastomer honeycomb body is elastically buckled and deformed, and meanwhile, the elastic-plastic honeycomb saw is plastically buckled through the local plastic hinge and the cell wall fracture. Elastomeric honeycombs and EPS foams exhibit very similar force-displacement curves, where force is proportional to displacement. However, elastoplastic honeycombs achieve a higher initial force maintained on the sample, which means that the impact energy is dissipated with a lower peak load for a shorter duration. Acceleration-time curves for three different mechanisms were analyzed by head performance index, where elastoplastic honeycombs reached the lowest values predicted for head damage and were within the wien state tolerance curve. When the energy absorption per volume (EPV) of peak acceleration is plotted, the EPV of the elastoplastic honeycomb is found to be 3 times higher than that of the expanded polystyrene helmet.

Test data sink

Appendix B-additive manufacturing techniques

The additive manufacturing technique provides a fast process of creating complex geometries, which would be impossible or very expensive compared to conventional subtraction/forming methods. The additive manufacturing technique works by building a computer-aided design directly from layer-by-layer build-up of material. Laser sintering is a form of incremental manufacturing technique whereby a thin layer of powder is deposited on a preheated build area, followed by the use of CO₂The laser selectively solidifies the powder. Laser sintering is chosen as the process for making hexagonal structures because of its high mechanical properties. Laser sintering remains a relatively young manufacturing technique requiring a specific thermal window to cure, so there are only a few material options. However, the microstructure can be altered by using a range of different processing conditions.

The mechanical properties of laser sintering can be attributed in part to the extent of the particle melt (DPM), which defines the amount of sintering consolidation change.

Appendix C-discussion of in-plane and out-of-plane mechanics

In-plane

During compression, the cell walls initially bend, creating linear elasticity. But when critical stress is reached, the cell begins to collapse: the collapse of the elastic material is recoverable due to elastic buckling of the cell walls; in materials with a plastic yield point, due to the formation of a plastic hinge at the maximum moment portion of the bending member; in brittle materials, due to brittle fracture of the cell walls; the latter two are unrecoverable.

Eventually, at high strain, the cell collapses sufficiently that the opposing cell walls come into contact (or the broken pieces pack together) and further deformation compresses the cell wall material itself. This gives the final, steeply rising portion of the stress-strain curve called densification.

Out-of-plane surface

During compression, the cell walls are initially compressed axially such that the young's modulus varies linearly with relative density, while the poisson's ratio is solid. In an elastomeric material, once the elastomer is unloaded, the honeycomb recovers its curvature (usually due to hysteresis effects due to energy passing through heat losses) and the cell walls bend. The plastic material has a yield point after which a permanent deformation occurs by means of a local plastic hinge (cell wall bending). Ceramic materials typically fail by cell wall rupture.

The honeycomb materials used to collect the test results were laser sintered viscoelastic polyamide and elastomer. The plasticity and fracture of the polymer are dependent on temperature and strain rate. At a lower temperature

The polymer is linear elastic rupture. At higher temperatures

The failure mode changes from brittle to plastic, characterized by the yield point. Failure mechanism diagrams are used to summarize plasticity and fracture response in amorphous polymers and elastomers, respectively.

For elastomers, the elastic buckling load is determined by the column equation for euler buckling if the cell walls are constrained parallel to the cell plane:

the constant K is an ending constraint factor, typically equal to 4. If the cell height is greater than I (> 31), then K is independent of the cell height. The single wall t will maintain the same load after reaching the initial collapse load PCrit, the total collapse load being 6PCrit divided by the loaded cross-sectional area:

where σ el is the elastic buckling stress and 5.2 is the value found from the cell geometry of the regular honeycomb, where v5 is assumed to be 0.3.

For elastoplastic materials, Wierzbicki found that the lowest plastic collapse strength (most likely to occur) in compression is due to plastic buckling. Plastic bending dissipates energy through permanent rotation of the cell walls. Wierzbicki yields an approximation based on the separation cell wall. The plastic failure stress of a regular hexagon with a uniform wall thickness t is:

wherein sigmaysIs the yield stress.

In brittle materials, if the net cross-sectional stress σ 3 exceeds the tensile rupture strength σ of the cell wall solidfsThe honeycomb will fail in tension. Brittle solids are stronger in compression than in tension because compressive stress closes small cracks or defects that ultimately determine strength. But even if they are closed, these defects can sever and the severing is concentrated on the stress in a manner that will still result in cracking. As a result, it was found experimentally that the crushing strength σ of the cell wall outside the planecrIs more than 12 times of the breaking strength.

A curved dominant structure such as foam is analyzed by an energy absorption map. The energy absorbed per unit volume W is given by the area under the stress-strain curve in the following graph (a).

In elastomeric materials, the failure mechanism is elastic buckling, so most of the energy is stored elastically. In plastic and brittle materials, energy is stored elastically up to the yield point, after which it is dissipated by plastic bending or cell wall rupture. In graph (a), there is little change in the peak stress σ ρ as we move along the strain axis, with an increase in the amount of energy absorbed W (dissipated or stored). During densification, the peak stress sharply rises and W hardly changes. The best use of the energy absorption properties of the foam is achieved by using the shoulders of the curve, i.e. absorbing as much energy as possible for a given peak stress. The envelopes of the shoulders for different foam densities are plotted in graph (b). The envelope describes the relationship between W and σ ρ for selecting the optimum relative density at a particular strain rate and temperature.

Modeling of the energy absorption map can be used for elastomers and elastoplastic materials with very small linear elastic regions. The modeling process for elastomers and elastoplastics is the same, the final equation being of the form:

where σ D is the densification stress, which is assumed to be at the same level as the plateau stress for the bend dominated structure.WmaxIs the maximum energy that can be absorbed. The equations developed above show

Only depend on

And

that is, the chart depicts all density and material properties of the foam.

Appendix D-Material Properties and hexagonal cell Structure optimization

The response of the honeycomb material is critical if used for energy absorption because it can withstand various impact velocity and temperature conditions. An investigation was conducted to understand the strain rate and temperature dependence under different laser sintering processing conditions. The material studied was polyamide 12, all tests being under compression.

Four Energy Densities (ED) (J/mm) were investigated²) As shown in the table above. The ED processing is controlled by the epsc innovative manufacturing center at nottingham university via an EOS P100 machine. The ED is varied by laser power and scanning speed. To obtain part uniformity, the orientation and position of each feature is uniform. The initial compression characteristic was performed through a series of strain rates to study the relationship of energy density in axial compression. One of the materials, ED2, was subjected to a pseudo-static compression test at a temperature of-60 ℃ to 60 ℃. All experiments were repeated three times and an average sample response was submitted. The mass and volume of each material produced at each energy density was measured five times to calculate the respective average physical density. The mass was measured using an analytical balance with readability of 0.01mg and the volume was measured using a micrometer with readability of 0.001 mm.

Low compression speed was performed using an Instron tester (0.001, 0.01 and 0.1 s)^-1). For these low strength materials, compliance of the machine is not an issue, and true strain control from the crosshead is used; however, an extensometer was also attached to the loading anvil close to the sample to verify specimen extensibility. The total compressive resistance on the sample as a function of time was obtained from a 100kN load cell with a standard accuracy of ± 0.05N. By use of for access 1 and 50 s^-1Strain rate in betweenTo achieve moderate strain rates (1 and 10 s)^-1). A linear differential transformer (LVDT) measures the displacement of the sample; the signal receives no significant distortion from ringing on the load cell and other instrument noise. High strain rate using Split Hopkinson Pressure Bar (SHPB) ((S))>1000s^-1) And (4) performing a compression experiment. For the SHPB system, the input and output rods are made of silver steel. The input rod is 1m long and is measured along half of its length; the output shaft is 500mm long and is measured from the sample shaft at 50 mm. In the case of using standard analysis, the stress-strain relationship is derived using the reflection and transmission measurement signals. Petrolatum is used as a lubricant. For non-ambient quasi-static experiments, nitrogen and a heating wire were used to obtain the necessary chamber temperature. Each sample was preheated/cooled at the test temperature for 5-10 minutes to ensure thermal equilibrium.

The high energy density samples were found to have rough surface areas, indicating large surface porosity. This porosity may weaken the material because of the lower volume fraction of solids present.

Appendix E-stretch dominant structural mechanics

One example of a tensile dominated structure is a micro-truss lattice or out-of-plane honeycomb where the deformation mechanism involves "hard" modes, such as compression and stretching, rather than bending. The lower graph shows the stress-strain curve of a tensile dominated structure with elastoplastic material. Yield stress occurs due to localized plastic buckling and brittle failure of the struts. This is also called the bifurcation point, as the structure becomes unstable and then a post-flexor softening scheme is created.

The stress rises sharply under densification strain, which can be calculated from the following formula:

the dorsiflexion softening offsets the increased area of the elliptical helmet that dissipates energy under more uniform plateau forces. For example, line x in the lower graph is the dorsiflexion softening seen in the experimental results, while line y represents the increase in area for a particular head shape. The constant plateau force is obtained by multiplying the stress reduction and the area increase, which helps to reduce the peak force and thickness required to absorb energy.

Since the tensile dominated structure relies on a hard deformation mode, the stress in yielding is much higher compared to foam (bending dominated structure). This can be seen in the following figure, where the relative modulus is the ratio between the strength of the matrix material and the structural strength at a particular relative density.

Thus, for a given yield stress, the relative density of the tensile dominated structure can be much lower. Densification strain is inversely proportional to relative density according to the equation below.

Where ρ is the structural density and ρ s is the density of the matrix material, where ρ_critIs the relative density (or solid volume fraction) of the structural lock, typically 0.71. The following figures describe the results of experiments confirming the above relationship.

Thus, for the desired yield stress, the tensile dominated structure requires a lower relative density and a greater densification strain is obtained according to the above equation. Since the amount of absorbed energy is the product of stress and strain, increasing strain will mean increasing the amount of absorbed energy (essentially less material is required for stretch dominated structures and therefore longer displacement before cell wall densification increases the absorbed energy).

Claims

1. A shock absorbing structure comprising a unitary material formed as a stretch-dominated hollow cell structure; wherein the hollow cell structure has a relative density of between 0.05 and 0.15, the relative density being defined by ρ/ρ_sDefinition, where ρ is the density of the hollow cell structure, ρ_sIs the density of the bulk material.

2. An impact absorbing structure as claimed in claim 1 wherein all cells of the hollow cell structure have a cross section which is uniform throughout the depth of the cell.

3. An impact absorbing structure as claimed in claim 1 wherein the cells are formed as a micro-truss lattice.

4. An impact absorbing structure as claimed in claim 1 wherein the cells are formed as a lattice structure.

5. An impact absorbing structure as claimed in any one of claims 1 to 2 wherein at least a plurality of the cells are configured as tessellations.

6. An impact absorbing structure as claimed in claim 5 wherein at least a plurality of the cells are configured to tessellate with a cell axis orthogonal to the top or bottom surface of the cell.

7. An impact absorbing structure as claimed in claim 6 wherein at least a plurality of the cells are hexagonal.

8. An impact absorbing structure as claimed in claim 6 wherein at least a plurality of the cells are triangular.

9. An impact absorbing structure as claimed in claim 6 wherein at least a plurality of the cells are square.

10. An impact absorbing structure as claimed in claim 6 wherein at least a plurality of the cells are a combination of octagons and squares co-located in an inlay tessellation.

11. An impact absorbing structure as claimed in claim 1 wherein the cell shape, size, cell wall thickness, cell width and cell length can be varied freely with respect to one another.

12. An impact absorbing structure as claimed in claim 10 or 11 wherein the cell walls have a maximum thickness of 1 mm.

13. An impact absorbing structure as claimed in claim 1 wherein the unitary material is a polymeric material.

14. An impact absorbing structure as claimed in claim 13 wherein the unitary material is an elastomer.

15. An impact absorbing structure as claimed in any one of claims 13 to 14 wherein the unitary material is nylon 11.

16. An impact absorbing structure as claimed in any one of claims 13 to 14 wherein the hollow cell structure is fabricated by laser sintering.

17. A helmet comprising an inner impact resistant liner formed at least in part on an impact absorbing structure as claimed in any one of claims 1 to 16.

18. The helmet of claim 17, further comprising an outer shell for covering the inner impact resistant liner.

19. The helmet of claim 18 wherein the outer shell is formed at least in part from a composite material.

20. The helmet of claim 19 wherein the outer shell is formed at least in part from a thermoplastic material.

21. A helmet according to any one of claims 18 to 20 wherein the outer shell is formed with at least one vent slot.

22. A method of optimizing a shock absorbing structure for improved shock absorption, comprising the steps of:

(i) selecting an integral material;

(ii) forming the integral material into a stretch-dominated hollow cell structure; wherein the hollow cell structure has a relative density of between 0.05 and 0.15, the relative density being defined by ρ/ρ_sDefinition, where p isDensity of empty cell structure, ρ_sIs the density of the bulk material.

23. A method of optimising an impact absorbing structure as claimed in claim 22 wherein all of the cells of the hollow cell structure have a cross-section which is uniform throughout the depth of the cell.

24. A method of optimising an impact absorbing structure as claimed in claim 23 wherein the cells are formed as a micro-truss lattice.

25. A method of optimising an impact absorbing structure as claimed in claim 23 wherein the cells are formed as a lattice structure.

26. A method of optimising an impact absorbing structure as claimed in any one of claims 22 to 25 wherein at least a plurality of the cells are formed so as to tessellate.

27. A method of optimising an impact absorbing structure as claimed in claim 26 wherein at least a plurality of the cells are formed so as to tessellate with a cell axis orthogonal to the top or bottom surface of the cell.

28. A method of optimising an impact absorbing structure as claimed in claim 27 wherein the hollow cells are formed to have a topology which is radially perpendicular to the curved surface.

29. A method of optimising an impact absorbing structure as claimed in any one of claims 27 to 28 wherein at least a plurality of the cells are formed as hexagons.

30. A method of optimising an impact absorbing structure as claimed in any one of claims 27 to 28 wherein at least a plurality of the cells are formed as triangles.

31. A method of optimising an impact absorbing structure as claimed in any one of claims 27 to 28 wherein at least a plurality of the cells are formed as squares.

32. A method of optimising an impact absorbing structure as claimed in any one of claims 27 to 28 wherein at least a plurality of the cells are formed as a combination of octagons and squares co-located in an tessellation pattern.

33. A method of optimising an impact absorbing structure as claimed in claim 22 wherein the cells are formed such that the cell shape, size, cell wall thickness, cell width and cell length may be varied freely with respect to one another.

34. A method of optimising an impact absorbing structure as claimed in claim 33 wherein the cells are formed such that the cell walls have a maximum thickness of 1 mm.

35. A method of optimising an impact absorbing structure as claimed in any one of claims 22 wherein the unitary material is a polymeric material.

36. A method of optimising an impact absorbing structure as claimed in claim 35 wherein the unitary material is an elastomer.

37. A method of optimising an impact absorbing structure as claimed in any one of claims 36 wherein the unitary material is nylon 11.

38. A method of optimising an impact absorbing structure as claimed in any one of claims 35 to 37 wherein the hollow cell structure is fabricated by laser sintering.